The present invention relates to antennas and more particularly to slot antennas.
A slot antenna an electrically conductive sheet or plate (e.g. aluminum, copper, or other conductive metal or alloy) that defines a slot where the conductor is missing. When the plate is driven as an antenna by a driving frequency, the slot radiates electromagnetic waves like a dipole antenna.
FIG. 1 shows a typical prior art slot antenna 10. The length of the slot 12 determines the optimum operating frequency of the slot antenna 10. The length of the slot 12 is approximately one-half of the wavelength of the optimum operating frequency. Each end of the slot has no electric field because the conductive material will not support a voltage potential. The center of the slot supports a high electric field. The variation of the electric field along the length of the slot has a corresponding impedance variation. The center of the slot supports a high voltage field (E-field) and a low magnetic field (B-field), so the impedance is high. Each end of the slot has a low E-field and a high B-field, so the impedance is low. A relatively narrow slot tends to decrease the capacitive reactance of the slot antenna 10, and a relatively wide slot tends to increase the capacitive reactance of the antenna.
Exciting the slot antenna is accomplished by establishing an alternating current (AC) voltage potential across the slot. The most efficient means of excitation is a power source with an impedance that is matched to the location of the feed. So, feeding across the center of the slot would require a high-impedance source, and feeding across other locations along the length of the slot would require lower-impedance sources. Typically, the feed point is located near one end of the slot so that the impedance is near the standard value of 50 ohms.
The AC voltage is applied across the slot 12 by way of the feed 14. By adjusting the location of the feed 14 along the length of the slot 12, the impedance of the antenna 10 can be matched to the impedance of the power source. The reactance of the slot may be matched to the reactance of the power source by varying the slot width.
While slot antennas have proven to be effective in many applications, the size required of a slot antenna limits the variety of applications in which such an antenna can be used, especially in view of the constant size reduction of products. Therefore, a slot antenna of reduced size is highly desirable.